The present invention relates to a holographic endoscope used for recording a hologram, and more particularly to a holographic endoscope wherein a field lens is disposed in front of a photographic plate on which an interference pattern is recorded.
As is now widely known, holographic techniques are employed for examinations or measurements of physical phenomena, such as the state of deformation of the surface of an object. There has been proposed a holographic endoscope which is used for recording the interference pattern of wave fronts of light from an object, such as a portion which is otherwise invisible, for example the inside of an engine or other apparatus, the cavity of a human body such as the stomach or intestine, or the like. The recorded information, i.e. a hologram, is then used to reconstruct an image of the subject for visible inspection. Otherwise, the reconstructed image may be recorded as an ordinal photograph for examinations or measurements.
A holographic endoscope which is well known is shown in FIG. 1, by way of example. In this holographic endoscope, two beams are directed from a single laser 1, such as an argon laser, by means of a beam splitter 2. One beam which is referred to as the illumination beam is reflected by a mirror M1 and used to illuminate an object I.sub.o. The illumination beam is guided through a light transmitter 3 or optical fiber bundle (which may be a means comprising a combination of lenses) which withdrawably extends through an insertable section of an endoscope and is extended therefrom to illuminate the object Io to be examined. The light reflected from the object I.sub.o (which is referred to as an "object light") bears an image of the object which is formed by an objective lens 4 on an end surface of an image-transmitter 5 or image-transmitting optical fiber bundle and then transmitted to the opposite end surface 5a thereof. The other beam is reflected by mirrors M2 and M3 and used as a reference beam. The reference beam and the object light are then allowed to interfere with each other, and the resulting interference pattern is recorded on a photographic plate 7, forming the hologram 9. A divergent lens 6 broadens the reference beam without affecting its coherence.
The hologram 9 thus formed and then developed by the process of ordinary photographic technique is illuminated by the reference beam alone, reproducing the same wave fronts as reflected from the original object. The reproduced wave fronts appear to emanate from an image I.sub.H of the subject. This image I.sub.H can then be viewed with the eye 11 or other optical instruments or recorded with a camera. For shielding the illumination beam upon illuminating the hologram 9 with the reference beam, there is provided a shutter 10 as shown in FIG. 1. It should be noted that the holographic endoscope is available as a conventional one in case the photographic plate 7 is removed, while guiding the illumination light emitted from a halogen or xenon lamp through the light transmitting optical fiber bundle 3. In this case, a virtual image of the image I1 formed by the lens 8 can be observed either visually or with a camera.
Such a holographic endoscope as described above, however, has a problem to be solved, that is, it becomes difficult to provide enough space for directing the reference beam to the photographic plate 7 with decreasing distance between the lens 8 and photographic plate 7. On the contrary, upon providing enough space therebetween, the object beam is broadened on the photographic plate with a result of a lowering of density of light energy per unit area, resulting in a prolonged exposure time for recording the interference pattern. The photographic plate under a long time exposure is very susceptible to vibrations, fluctuations of air and the like, so that the contrast of interference pattern recorded becomes low, resulting in a blur and indiscernible reconstructed image.
In addition, there is caused in the aforementioned holographic endoscope another problem, namely that what one can visually observe from point is only a part of the whole reconstructed image IH, and in order to observe the whole reconstructed image IH, it is necessary to move the eye 11 back and forth, and right and left, since the object beam is broadened through the lens 8 and the distance D2 between the lens 8 and the eye 11 is long.
In view of the actual circumstances, such a holographic endoscope as is shown in FIG. 2 has been proposed. In FIG. 2, wherein the same numerals and symbols denote similar parts to those in FIG. 1, the image I1 transmitted to the end of the image-transmitting optical fiber bundle 5 is magnified by the lens 8 to form an image I2. A photographic plate 20 on which an interference pattern is recorded should be disposed between the lens 8 and the magnified image I2.
The holographic endoscope thus constructed is able to provide a high density of light energy per unit area by adjusting the axial position of the photographic plate 20. As a result of this, a high contrast and bright reconstructed image is obtained.
In this case, however, it becomes difficult to direct the reference beam broadened through the lens 6 to the photographic plate 20 because the photographic plate 20 is brought close to the lens 8 for providing a high density of light energy. Additionally, the reconstructed image I2, which is visible therebehind, is only partially observable and when it is necessary to observe the whole, an optical system 21 for observation is indispensable.